FELECTPODA 



563 



C. reticulata (L.) (Fig. 887) . Shell 5.5 cm. long and banded with yel- 

 low, white, and brown : Cape Hatteras to west Florida, in shallow water. 



CLASS 4. PELECYPODA.* (LAMELLIBRANCHIATA.) 



Symmetrical mollusks with a double or bivalve shell and mantle, 

 and without a head (Fig. 740, C). 



External Structure. The compact visceral mass is compressed later- 

 ally and the two lobes of the mantle extending downwards from the 

 back entirely enclose both it and the foot. When the animal moves 

 about the foot is extended between the edges of the mantle (Fig. 888, 1). 

 The hinder part of the mantle lobes controls the inflow and the outflow 

 of water between the 

 mantle cavity and the out- A 



side. In some of the more 

 primitive bivalves, espe- 

 cially such as are sessile, 

 as the oyster, this func- 

 tion is but little localized, 

 a current flowing out of 

 the mantle cavity in the 

 region of the anus and one 

 flowing into it at other 

 portions of its hinder and 

 lower borders (Fig. 888). 

 In most bivalves, how- 

 ever, the hinder margins 

 of the mantle are highly 

 modified, and are closely 



applied to each other, leaving two openings which are called siphons; 

 through the lowermost of these, which is called the branchial or 

 incurrent siphon, water flows into the mantle cavity, bringing the 

 microscopic food of the animal and the air needed for respiration, and 

 through the uppermost, which is called the cloacal or excurrent siphon, 

 it flows out, conveying the exhausted respiratory water with the excre- 

 ment and often the genital products. In very many pelecypods the 

 posterior mantle edges forming the siphons are not merely applied to 

 each other but are fused together and extended in the form of tubes. 

 Fusion has also taken place to a greater or less extent along the entire 

 edge of the mantle in the higher bivalves, a small slit only being left 

 for the foot (Fig. 888). 



* See "The Mollusca of the Chicago Area ; The Pclecypoda," by F. C. Baker, 

 The Natural History Survey, Bull. No. Ill, Pt. 1, 1898. 



Fig. 888 Diagrams showing the formation of the 

 siphons, the anterior end in each figure being 

 towards the left and the dorsal side being above. 

 A, mantle open and no definite siphons ; B, mantle 

 open and incomplete siphons ; C, cloacal siphon 

 complete ; D, both siphons complete but not ex- 

 tended ; E, siphons extended and mantles joined 

 ventrally ; F, mantles completely joined, leaving a 

 narrow space for the foot and one for the byssus 

 (Lang). 1, foot; 2, left-hand mantle; 3, right- 

 hand mantle ; 4, cloacal siphon ; 5, branchial 

 siphon ; 6, byssus opening. 



